Generally, a magnetic recording medium is produced by coating a magnetic paint comprising a ferromagnetic powder, a binder, and one or more other components on a nonmagnetic support.
In recent years, there have been increasing demands for higher recording density for magnetic recording media, particularly video recording media, which necessitate recording in a high frequency range. With this trend toward higher recording density, a finer ferromagnetic powder having a higher coercive force tends to be used, which has led to the use of magnetic powders having a specific surface area exceeding 45 m.sup.2 /g.
However, the finer and more coercive the ferromagnetic powders become, the more coagulative the grains become, which results in difficulties in obtaining sufficient dispersibility of the grains and a surface smoothness of a medium necessary to ensure a high reproduction output of high frequency recording and a good S/N ratio. This tendency is especially noticeable in a ferromagnetic metal powder. Also, a magnetic recording medium containing an insufficiently dispersed ferromagnetic powder has a shortcoming that the magnetic powder contained therein is likely to fall during recording and reproducing, which may cause magnetic head clogging and other undesirable problems and thus deteriorate a running durability.
Various methods of obtaining improved dispersibility have so far been proposed as listed in Table 1 below. In the majority of these methods, kneading is carried out in two stages to prepare a magnetic paint. In this table, "metal" means a magnetic metal powder and "vinyl chloride/acetate" means a vinyl chloride-vinyl acetate copolymer.
TABLE 1 __________________________________________________________________________ Reference First Stage Second stage __________________________________________________________________________ JP O.P.I. No. 1677769/1981 Nitrocellulose Kneader Polyurethane + abrasive Ball mill JP O.P.I. No. 167770/1981 Nitrocellulose Kneader Polyurethane + lubricant Ball mill JP O.P.I. No. 167771/1981 Polyurethane + metal Kneader Vinyl chloride/acetate + Ball mill JP O.P.I. No. 167772/1981 Polyurethane + metal Kneader Vinyl chloride/acetate + Ball mill abrasive JP O.P.I. No. 138732/1985 Vinyl chloride/ Kneader Polyurethane Sand mill acetate + metal JP O.P.I. No. 8726/1986 Metal Kneader Sand mill JP O.P.I. No. 187931/1985 Kneader Lubricant Sand mill JP O.P.I. No. 125920/1985 Vinyl chloride/acetate Kneader Polyurethane Sand mill JP O.P.I. No. 200423/1983 Metal + abrasive + Kneader Binder 2 Kneader binder 1 JP O.P.I. No. 151822/1985 Vinyl chloride/acetate Kneader Polyurethane Sand mill (modified with maleic acid) JP O.P.I. No. 141026/1982 Polyurethane + metal Kneader Vinyl chloride/acetate Sand mill JP O.P.I. No. 46520/1979 Vinyl chloride/acetate Kneader Sand mill JP O.P.I. No. 76012/1978 Kneader Sand mill __________________________________________________________________________ JP O.P.I.: Japanese Patent Publication Open to Public Inspection
These conventional methods have the following problems:
(1) If kneading in the second stage is not carried out, insufficient dispersibility will lead to the problems related to electromagnetic conversion characteristics and a running durability. PA0 (2) Where polyurethane is kneaded in the first stage and vinyl chloride/acetate is kneaded in the second stage together with a lubricant and an abrasive, a dispersibility is deteriorated. Also, since the polyurethane kneaded in the first stage has an insufficient affinity to a magnetic metal powder, a kneading effect is unsatisfactory and the prescribed electromagnetic conversion properties and a running durability cannot be obtained. Vinyl chloride/acetate added in the second stage will give similar results (JP O.P.I. Nos. 167771/1981 and 167772/1981). PA0 (3) Where vinyl chloride/acetate in the second stage is not kneaded with a kneader unlike polyurethane in the first stage, vinyl chloride/acetate can not be sufficiently affinitive to a magnetic metal powder, so that a stability of a coating solution after dispersion is lowered and the electromagnetic conversion properties are deteriorated (JP O.P.I. No. 141026/1982). PA0 (4) Where the binders (polyurethane and vinyl chloride/acetate) are kneaded in a single stage, a coating solution viscosity after dilution and dispersion is so high that a gloss, the electromagnetic conversion properties and a running durability of a recording medium becomes poor. PA0 (5) Where an abrasive is added in kneading, it will adversely affect dispersion of a magnetic powder (magnetic powder damage due to a higher hardness of the abrasive), or poor dispersion of the abrasive attributable to difference of the optimum dispersion point of the abrasive itself will make it easy for a magnetic layer to drop off after coating and to cause a dropout problem (JP O.P.I. No. 200423/1983). PA0 (6) Where a binder added in kneading is vinyl chloride/acetate and polyurethane is used in dispersing, polyurethane is not sufficiently dispersed, so that a running durability and the electromagnetic conversion properties are deteriorated (JP O.P.I. Nos. 138732/1985 and 8726/1986). PA0 (a) the first kneading process in which 100 parts by weight of a ferromagnetic powder, 3 to 15 parts by weight of at least one of polyurethane and polyester, and 10 to 60 parts by weight of a solvent are kneaded under the load of the power consumption of 0.05 to 0.5 kW per kg of the ferromagnetic powder; and PA0 (b) the second kneading process in which the mixture prepared in the first kneading process is further kneaded under the load of the power consumption of 0.05 to 0.5 kW per kg of the ferromagnetic powder after adding 3 to 15 parts by weight of a vinyl chloride resin.